The present invention relates to a manufacturing method for a multilayered gas sensor element to be incorporated in an exhaust gas system for an internal combustion engine to control the combustion of the engine.
Multilayered gas sensing elements are preferably installed in exhaust gas systems of automotive engines to measure various gas concentrations and control the combustion of the engines, and are arranged in the following manner.
For example, as disclosed in the Japanese Patent Application Laid-open No. 2001-66280, a conventional gas sensing element includes a solid electrolytic substance. A measured gas side electrode is provided on one surface of this solid electrolytic substance so as to be exposed to a measured gas. A porous diffusion resistance layer covers the outer surface of the measured gas side electrode. And, a dense protection layer covers the outer surface of the diffusion resistance layer. Furthermore, a reference electrode is provided on the other surface of the solid electrolytic substance so as to be exposed to a reference gas.
The above-described reference electrode is provided in a reference gas chamber into which the reference gas is introduced. The reference gas chamber is defined by laminating a spacer on the solid electrolytic substance. A heater portion is integrally laminated with the spacer. The heater portion consists of a heat generating element which generates heat in response to supply of electric power and a heater substrate provided with lead portions and terminal portions which are electrically connected to the heat generating element.
The multilayered gas sensor element, as described later in a preferred embodiment of the present invention, can be manufactured by sintering a laminated body of green sheets being appropriately laminated to constitute a diffusion resistance layer, a protection layer, a solid electrolytic substance, a spacer, and a heater substrate. However, as shown in FIG. 4, surface defects 195 such as chips and cracks tend to appear on side surfaces 193 and 194 and element surfaces 191 and 192 of a multilayered gas sensor element 1 as a result of sintering operation. Furthermore, there is the possibility that warpage appears on the side surfaces 191 and 192 of the multilayered gas sensor element 1 (refer to FIG. 4). Undesirable differences in altitudinal level may also be caused due to lamination error.
The above-described surface defects 195 may be not so large to give adverse influence to the operation of multilayered gas sensor element 1, for example, immediately after finishing the sintering operation. However, as shown in FIG. 5, this kind of surface defects 195 possibly grow into larger defects when electric power is supplied to a heater portion 18 and will stretch into the inner portion of the of the multilayered gas sensor element 1.
Accordingly, reducing such undesirable surface defects as much as possible is very important to improve the production yield in the manufacturing of the multilayered gas sensor elements.